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\begin{document} |
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|
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\section{Introduction} |
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Interfacial thermal conductance is extensively studied both |
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experimentally and computationally, due to its importance in nanoscale |
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science and technology. Reliability of nanoscale devices depends on |
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their thermal transport properties. Unlike bulk homogeneous materials, |
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nanoscale materials features significant presence of interfaces, and |
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these interfaces could dominate the heat transfer behavior of these |
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experimentally and computationally\cite{cahill:793}, due to its |
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importance in nanoscale science and technology. Reliability of |
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nanoscale devices depends on their thermal transport |
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properties. Unlike bulk homogeneous materials, nanoscale materials |
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features significant presence of interfaces, and these interfaces |
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could dominate the heat transfer behavior of these |
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|
materials. Furthermore, these materials are generally heterogeneous, |
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which challenges traditional research methods for homogeneous systems. |
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which challenges traditional research methods for homogeneous |
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systems. |
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|
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Heat conductance of molecular and nano-scale interfaces will be |
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affected by the chemical details of the surface. Experimentally, |
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that specific ligands (capping agents) could completely eliminate this |
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barrier ($G\rightarrow\infty$)\cite{doi:10.1021/la904855s}. |
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|
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Theoretical and computational studies were also engaged in the |
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interfacial thermal transport research in order to gain an |
104 |
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understanding of this phenomena at the molecular level. However, the |
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relatively low thermal flux through interfaces is difficult to measure |
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with EMD or forward NEMD simulation methods. Therefore, developing |
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good simulation methods will be desirable in order to investigate |
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thermal transport across interfaces. |
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Theoretical and computational models have also been used to study the |
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interfacial thermal transport in order to gain an understanding of |
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this phenomena at the molecular level. Recently, Hase and coworkers |
105 |
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employed Non-Equilibrium Molecular Dynamics (NEMD) simulations to |
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study thermal transport from hot Au(111) substrate to a self-assembled |
107 |
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monolayer of alkylthiol with relatively long chain (8-20 carbon |
108 |
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atoms)\cite{hase:2010,hase:2011}. However, ensemble averaged |
109 |
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measurements for heat conductance of interfaces between the capping |
110 |
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monolayer on Au and a solvent phase has yet to be studied. |
111 |
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The comparatively low thermal flux through interfaces is |
112 |
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difficult to measure with Equilibrium MD or forward NEMD simulation |
113 |
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methods. Therefore, the Reverse NEMD (RNEMD) methods would have the |
114 |
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advantage of having this difficult to measure flux known when studying |
115 |
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the thermal transport across interfaces, given that the simulation |
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methods being able to effectively apply an unphysical flux in |
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non-homogeneous systems. |
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|
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Recently, we have developed the Non-Isotropic Velocity Scaling (NIVS) |
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algorithm for RNEMD simulations\cite{kuang:164101}. This algorithm |
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retains the desirable features of RNEMD (conservation of linear |
122 |
|
momentum and total energy, compatibility with periodic boundary |
123 |
|
conditions) while establishing true thermal distributions in each of |
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the two slabs. Furthermore, it allows more effective thermal exchange |
125 |
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between particles of different identities, and thus enables extensive |
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study of interfacial conductance. |
124 |
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the two slabs. Furthermore, it allows effective thermal exchange |
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between particles of different identities, and thus makes the study of |
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interfacial conductance much simpler. |
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|
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[BRIEF INTRO OF OUR PAPER] |
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[WHY STUDY AU-THIOL SURFACE][CITE SHAOYI JIANG] |
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The work presented here deals with the Au(111) surface covered to |
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varying degrees by butanethiol, a capping agent with short carbon |
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chain, and solvated with organic solvents of different molecular |
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properties. Different models were used for both the capping agent and |
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the solvent force field parameters. Using the NIVS algorithm, the |
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thermal transport across these interfaces was studied and the |
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underlying mechanism for this phenomena was investigated. |
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|
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[MAY ADD WHY STUDY AU-THIOL SURFACE; CITE SHAOYI JIANG] |
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|
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|
\section{Methodology} |
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\subsection{Algorithm} |
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[BACKGROUND FOR MD METHODS] |
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There have been many algorithms for computing thermal conductivity |
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using molecular dynamics simulations. However, interfacial conductance |
143 |
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is at least an order of magnitude smaller. This would make the |
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calculation even more difficult for those slowly-converging |
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equilibrium methods. Imposed-flux non-equilibrium |
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\subsection{Imposd-Flux Methods in MD Simulations} |
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For systems with low interfacial conductivity one must have a method |
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capable of generating relatively small fluxes, compared to those |
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required for bulk conductivity. This requirement makes the calculation |
143 |
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even more difficult for those slowly-converging equilibrium |
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methods\cite{Viscardy:2007lq}. |
145 |
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Forward methods impose gradient, but in interfacail conditions it is |
146 |
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not clear what behavior to impose at the boundary... |
147 |
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Imposed-flux reverse non-equilibrium |
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methods\cite{MullerPlathe:1997xw} have the flux set {\it a priori} and |
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the response of temperature or momentum gradients are easier to |
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measure than the flux, if unknown, and thus, is a preferable way to |
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the forward NEMD methods. Although the momentum swapping approach for |
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flux-imposing can be used for exchanging energy between particles of |
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different identity, the kinetic energy transfer efficiency is affected |
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by the mass difference between the particles, which limits its |
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application on heterogeneous interfacial systems. |
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the thermal response becomes easier to |
150 |
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measure than the flux. Although M\"{u}ller-Plathe's original momentum |
151 |
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swapping approach can be used for exchanging energy between particles |
152 |
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of different identity, the kinetic energy transfer efficiency is |
153 |
> |
affected by the mass difference between the particles, which limits |
154 |
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its application on heterogeneous interfacial systems. |
155 |
|
|
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The non-isotropic velocity scaling (NIVS)\cite{kuang:164101} approach in |
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non-equilibrium MD simulations is able to impose relatively large |
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kinetic energy flux without obvious perturbation to the velocity |
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distribution of the simulated systems. Furthermore, this approach has |
156 |
> |
The non-isotropic velocity scaling (NIVS)\cite{kuang:164101} approach to |
157 |
> |
non-equilibrium MD simulations is able to impose a wide range of |
158 |
> |
kinetic energy fluxes without obvious perturbation to the velocity |
159 |
> |
distributions of the simulated systems. Furthermore, this approach has |
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the advantage in heterogeneous interfaces in that kinetic energy flux |
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can be applied between regions of particles of arbitary identity, and |
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the flux quantity is not restricted by particle mass difference. |
162 |
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the flux will not be restricted by difference in particle mass. |
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|
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The NIVS algorithm scales the velocity vectors in two separate regions |
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of a simulation system with respective diagonal scaling matricies. To |
166 |
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determine these scaling factors in the matricies, a set of equations |
167 |
|
including linear momentum conservation and kinetic energy conservation |
168 |
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constraints and target momentum/energy flux satisfaction is |
169 |
< |
solved. With the scaling operation applied to the system in a set |
170 |
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frequency, corresponding momentum/temperature gradients can be built, |
171 |
< |
which can be used for computing transportation properties and other |
172 |
< |
applications related to momentum/temperature gradients. The NIVS |
153 |
< |
algorithm conserves momenta and energy and does not depend on an |
154 |
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external thermostat. |
168 |
> |
constraints and target energy flux satisfaction is solved. With the |
169 |
> |
scaling operation applied to the system in a set frequency, bulk |
170 |
> |
temperature gradients can be easily established, and these can be used |
171 |
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for computing thermal conductivities. The NIVS algorithm conserves |
172 |
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momenta and energy and does not depend on an external thermostat. |
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|
|
174 |
|
\subsection{Defining Interfacial Thermal Conductivity $G$} |
175 |
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For interfaces with a relatively low interfacial conductance, the bulk |
187 |
|
T_\mathrm{cold}\rangle}$ are the average observed temperature of the |
188 |
|
two separated phases. |
189 |
|
|
190 |
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When the interfacial conductance is {\it not} small, two ways can be |
191 |
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used to define $G$. |
190 |
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When the interfacial conductance is {\it not} small, there are two |
191 |
> |
ways to define $G$. |
192 |
|
|
193 |
< |
One way is to assume the temperature is discretely different on two |
194 |
< |
sides of the interface, $G$ can be calculated with the thermal flux |
195 |
< |
applied $J$ and the maximum temperature difference measured along the |
196 |
< |
thermal gradient max($\Delta T$), which occurs at the interface, as: |
193 |
> |
One way is to assume the temperature is discrete on the two sides of |
194 |
> |
the interface. $G$ can be calculated using the applied thermal flux |
195 |
> |
$J$ and the maximum temperature difference measured along the thermal |
196 |
> |
gradient max($\Delta T$), which occurs at the Gibbs deviding surface, |
197 |
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as: |
198 |
|
\begin{equation} |
199 |
|
G=\frac{J}{\Delta T} |
200 |
|
\label{discreteG} |
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|
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With the temperature profile obtained from simulations, one is able to |
217 |
|
approximate the first and second derivatives of $T$ with finite |
218 |
< |
difference method and thus calculate $G^\prime$. |
218 |
> |
difference methods and thus calculate $G^\prime$. |
219 |
|
|
220 |
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In what follows, both definitions are used for calculation and comparison. |
220 |
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In what follows, both definitions have been used for calculation and |
221 |
> |
are compared in the results. |
222 |
|
|
223 |
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[IMPOSE G DEFINITION INTO OUR SYSTEMS] |
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To facilitate the use of the above definitions in calculating $G$ and |
225 |
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$G^\prime$, we have a metal slab with its (111) surfaces perpendicular |
226 |
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to the $z$-axis of our simulation cells. With or withour capping |
227 |
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agents on the surfaces, the metal slab is solvated with organic |
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solvents, as illustrated in Figure \ref{demoPic}. |
223 |
> |
To compare the above definitions ($G$ and $G^\prime$), we have modeled |
224 |
> |
a metal slab with its (111) surfaces perpendicular to the $z$-axis of |
225 |
> |
our simulation cells. Both with and withour capping agents on the |
226 |
> |
surfaces, the metal slab is solvated with simple organic solvents, as |
227 |
> |
illustrated in Figure \ref{demoPic}. |
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|
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|
\begin{figure} |
230 |
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\includegraphics[width=\linewidth]{demoPic} |
231 |
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\caption{A sample showing how a metal slab has its (111) surface |
232 |
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covered by capping agent molecules and solvated by hexane.} |
230 |
> |
\includegraphics[width=\linewidth]{method} |
231 |
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\caption{Interfacial conductance can be calculated by applying an |
232 |
> |
(unphysical) kinetic energy flux between two slabs, one located |
233 |
> |
within the metal and another on the edge of the periodic box. The |
234 |
> |
system responds by forming a thermal response or a gradient. In |
235 |
> |
bulk liquids, this gradient typically has a single slope, but in |
236 |
> |
interfacial systems, there are distinct thermal conductivity |
237 |
> |
domains. The interfacial conductance, $G$ is found by measuring the |
238 |
> |
temperature gap at the Gibbs dividing surface, or by using second |
239 |
> |
derivatives of the thermal profile.} |
240 |
|
\label{demoPic} |
241 |
|
\end{figure} |
242 |
|
|
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< |
With a simulation cell setup following the above manner, one is able |
244 |
< |
to equilibrate the system and impose an unphysical thermal flux |
245 |
< |
between the liquid and the metal phase with the NIVS algorithm. Under |
246 |
< |
a stablized thermal gradient induced by periodically applying the |
247 |
< |
unphysical flux, one is able to obtain a temperature profile and the |
248 |
< |
physical thermal flux corresponding to it, which equals to the |
249 |
< |
unphysical flux applied by NIVS. These data enables the evaluation of |
250 |
< |
the interfacial thermal conductance of a surface. Figure \ref{gradT} |
225 |
< |
is an example how those stablized thermal gradient can be used to |
226 |
< |
obtain the 1st and 2nd derivatives of the temperature profile. |
243 |
> |
With the simulation cell described above, we are able to equilibrate |
244 |
> |
the system and impose an unphysical thermal flux between the liquid |
245 |
> |
and the metal phase using the NIVS algorithm. By periodically applying |
246 |
> |
the unphysical flux, we are able to obtain a temperature profile and |
247 |
> |
its spatial derivatives. These quantities enable the evaluation of the |
248 |
> |
interfacial thermal conductance of a surface. Figure \ref{gradT} is an |
249 |
> |
example how those applied thermal fluxes can be used to obtain the 1st |
250 |
> |
and 2nd derivatives of the temperature profile. |
251 |
|
|
252 |
|
\begin{figure} |
253 |
|
\includegraphics[width=\linewidth]{gradT} |
256 |
|
\label{gradT} |
257 |
|
\end{figure} |
258 |
|
|
235 |
– |
[MAY INCLUDE POWER SPECTRUM PROTOCOL] |
236 |
– |
|
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|
\section{Computational Details} |
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|
\subsection{Simulation Protocol} |
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In our simulations, Au is used to construct a metal slab with bare |
262 |
< |
(111) surface perpendicular to the $z$-axis. Different slab thickness |
263 |
< |
(layer numbers of Au) are simulated. This metal slab is first |
264 |
< |
equilibrated under normal pressure (1 atm) and a desired |
265 |
< |
temperature. After equilibration, butanethiol is used as the capping |
266 |
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agent molecule to cover the bare Au (111) surfaces evenly. The sulfur |
267 |
< |
atoms in the butanethiol molecules would occupy the three-fold sites |
268 |
< |
of the surfaces, and the maximal butanethiol capacity on Au surface is |
269 |
< |
$1/3$ of the total number of surface Au atoms[CITATION]. A series of |
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different coverage surfaces is investigated in order to study the |
271 |
< |
relation between coverage and conductance. |
261 |
> |
The NIVS algorithm has been implemented in our MD simulation code, |
262 |
> |
OpenMD\cite{Meineke:2005gd,openmd}, and was used for our |
263 |
> |
simulations. Different slab thickness (layer numbers of Au) were |
264 |
> |
simulated. Metal slabs were first equilibrated under atmospheric |
265 |
> |
pressure (1 atm) and a desired temperature (e.g. 200K). After |
266 |
> |
equilibration, butanethiol capping agents were placed at three-fold |
267 |
> |
sites on the Au(111) surfaces. The maximum butanethiol capacity on Au |
268 |
> |
surface is $1/3$ of the total number of surface Au |
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> |
atoms\cite{vlugt:cpc2007154}. A series of different coverages was |
270 |
> |
investigated in order to study the relation between coverage and |
271 |
> |
interfacial conductance. |
272 |
|
|
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[COVERAGE DISCRIPTION] However, since the interactions between surface |
274 |
< |
Au and butanethiol is non-bonded, the capping agent molecules are |
275 |
< |
allowed to migrate to an empty neighbor three-fold site during a |
276 |
< |
simulation. Therefore, the initial configuration would not severely |
277 |
< |
affect the sampling of a variety of configurations of the same |
278 |
< |
coverage, and the final conductance measurement would be an average |
279 |
< |
effect of these configurations explored in the simulations. [MAY NEED FIGURES] |
273 |
> |
The capping agent molecules were allowed to migrate during the |
274 |
> |
simulations. They distributed themselves uniformly and sampled a |
275 |
> |
number of three-fold sites throughout out study. Therefore, the |
276 |
> |
initial configuration would not noticeably affect the sampling of a |
277 |
> |
variety of configurations of the same coverage, and the final |
278 |
> |
conductance measurement would be an average effect of these |
279 |
> |
configurations explored in the simulations. [MAY NEED FIGURES] |
280 |
|
|
281 |
< |
After the modified Au-butanethiol surface systems are equilibrated |
282 |
< |
under canonical ensemble, Packmol\cite{packmol} is used to pack |
283 |
< |
organic solvent molecules in the previously vacuum part of the |
284 |
< |
simulation cells, which guarantees that short range repulsive |
285 |
< |
interactions do not disrupt the simulations. Two solvents are |
286 |
< |
investigated, one which has little vibrational overlap with the |
265 |
< |
alkanethiol and plane-like shape (toluene), and one which has similar |
266 |
< |
vibrational frequencies and chain-like shape ({\it n}-hexane). [MAY |
267 |
< |
EXPLAIN WHY WE CHOOSE THEM] |
281 |
> |
After the modified Au-butanethiol surface systems were equilibrated |
282 |
> |
under canonical ensemble, organic solvent molecules were packed in the |
283 |
> |
previously empty part of the simulation cells\cite{packmol}. Two |
284 |
> |
solvents were investigated, one which has little vibrational overlap |
285 |
> |
with the alkanethiol and a planar shape (toluene), and one which has |
286 |
> |
similar vibrational frequencies and chain-like shape ({\it n}-hexane). |
287 |
|
|
288 |
< |
The spacing filled by solvent molecules, i.e. the gap between |
288 |
> |
The space filled by solvent molecules, i.e. the gap between |
289 |
|
periodically repeated Au-butanethiol surfaces should be carefully |
290 |
|
chosen. A very long length scale for the thermal gradient axis ($z$) |
291 |
|
may cause excessively hot or cold temperatures in the middle of the |
330 |
|
same type of particles and between particles of different species. |
331 |
|
|
332 |
|
The Au-Au interactions in metal lattice slab is described by the |
333 |
< |
quantum Sutton-Chen (QSC) formulation.\cite{PhysRevB.59.3527} The QSC |
333 |
> |
quantum Sutton-Chen (QSC) formulation\cite{PhysRevB.59.3527}. The QSC |
334 |
|
potentials include zero-point quantum corrections and are |
335 |
|
reparametrized for accurate surface energies compared to the |
336 |
|
Sutton-Chen potentials\cite{Chen90}. |
337 |
|
|
338 |
< |
Figure [REF] demonstrates how we name our pseudo-atoms of the |
339 |
< |
molecules in our simulations. |
321 |
< |
[FIGURE FOR MOLECULE NOMENCLATURE] |
338 |
> |
Figure \ref{demoMol} demonstrates how we name our pseudo-atoms of the |
339 |
> |
organic solvent molecules in our simulations. |
340 |
|
|
341 |
+ |
\begin{figure} |
342 |
+ |
\includegraphics[width=\linewidth]{structures} |
343 |
+ |
\caption{Structures of the capping agent and solvents utilized in |
344 |
+ |
these simulations. The chemically-distinct sites (a-e) are expanded |
345 |
+ |
in terms of constituent atoms for both United Atom (UA) and All Atom |
346 |
+ |
(AA) force fields. Most parameters are from |
347 |
+ |
Refs. \protect\cite{TraPPE-UA.alkanes,TraPPE-UA.alkylbenzenes} (UA) and |
348 |
+ |
\protect\cite{OPLSAA} (AA). Cross-interactions with the Au atoms are given |
349 |
+ |
in Table \ref{MnM}.} |
350 |
+ |
\label{demoMol} |
351 |
+ |
\end{figure} |
352 |
+ |
|
353 |
|
For both solvent molecules, straight chain {\it n}-hexane and aromatic |
354 |
|
toluene, United-Atom (UA) and All-Atom (AA) models are used |
355 |
|
respectively. The TraPPE-UA |
381 |
|
parameters for alkyl thiols. However, alkyl thiols adsorbed on Au(111) |
382 |
|
surfaces do not have the hydrogen atom bonded to sulfur. To adapt this |
383 |
|
change and derive suitable parameters for butanethiol adsorbed on |
384 |
< |
Au(111) surfaces, we adopt the S parameters from [CITATION CF VLUGT] |
385 |
< |
and modify parameters for its neighbor C atom for charge balance in |
386 |
< |
the molecule. Note that the model choice (UA or AA) of capping agent |
387 |
< |
can be different from the solvent. Regardless of model choice, the |
388 |
< |
force field parameters for interactions between capping agent and |
389 |
< |
solvent can be derived using Lorentz-Berthelot Mixing Rule: |
384 |
> |
Au(111) surfaces, we adopt the S parameters from Luedtke and |
385 |
> |
Landman\cite{landman:1998} and modify parameters for its neighbor C |
386 |
> |
atom for charge balance in the molecule. Note that the model choice |
387 |
> |
(UA or AA) of capping agent can be different from the |
388 |
> |
solvent. Regardless of model choice, the force field parameters for |
389 |
> |
interactions between capping agent and solvent can be derived using |
390 |
> |
Lorentz-Berthelot Mixing Rule: |
391 |
> |
\begin{eqnarray} |
392 |
> |
\sigma_{ij} & = & \frac{1}{2} \left(\sigma_{ii} + \sigma_{jj}\right) \\ |
393 |
> |
\epsilon_{ij} & = & \sqrt{\epsilon_{ii}\epsilon_{jj}} |
394 |
> |
\end{eqnarray} |
395 |
|
|
361 |
– |
|
396 |
|
To describe the interactions between metal Au and non-metal capping |
397 |
|
agent and solvent particles, we refer to an adsorption study of alkyl |
398 |
|
thiols on gold surfaces by Vlugt {\it et |
399 |
|
al.}\cite{vlugt:cpc2007154} They fitted an effective Lennard-Jones |
400 |
|
form of potential parameters for the interaction between Au and |
401 |
|
pseudo-atoms CH$_x$ and S based on a well-established and widely-used |
402 |
< |
effective potential of Hautman and Klein[CITATION] for the Au(111) |
403 |
< |
surface. As our simulations require the gold lattice slab to be |
404 |
< |
non-rigid so that it could accommodate kinetic energy for thermal |
402 |
> |
effective potential of Hautman and Klein\cite{hautman:4994} for the |
403 |
> |
Au(111) surface. As our simulations require the gold lattice slab to |
404 |
> |
be non-rigid so that it could accommodate kinetic energy for thermal |
405 |
|
transport study purpose, the pair-wise form of potentials is |
406 |
|
preferred. |
407 |
|
|
420 |
|
\begin{table*} |
421 |
|
\begin{minipage}{\linewidth} |
422 |
|
\begin{center} |
423 |
< |
\caption{Lennard-Jones parameters for Au-non-Metal |
424 |
< |
interactions in our simulations.} |
425 |
< |
|
426 |
< |
\begin{tabular}{ccc} |
423 |
> |
\caption{Non-bonded interaction parameters (including cross |
424 |
> |
interactions with Au atoms) for both force fields used in this |
425 |
> |
work.} |
426 |
> |
\begin{tabular}{lllllll} |
427 |
|
\hline\hline |
428 |
< |
Non-metal atom & $\sigma$ & $\epsilon$ \\ |
429 |
< |
(or pseudo-atom) & \AA & kcal/mol \\ |
428 |
> |
& Site & $\sigma_{ii}$ & $\epsilon_{ii}$ & $q_i$ & |
429 |
> |
$\sigma_{Au-i}$ & $\epsilon_{Au-i}$ \\ |
430 |
> |
& & (\AA) & (kcal/mol) & ($e$) & (\AA) & (kcal/mol) \\ |
431 |
|
\hline |
432 |
< |
S & 2.40 & 8.465 \\ |
433 |
< |
CH3 & 3.54 & 0.2146 \\ |
434 |
< |
CH2 & 3.54 & 0.1749 \\ |
435 |
< |
CT3 & 3.365 & 0.1373 \\ |
436 |
< |
CT2 & 3.365 & 0.1373 \\ |
437 |
< |
CTT & 3.365 & 0.1373 \\ |
438 |
< |
HC & 2.865 & 0.09256 \\ |
439 |
< |
CHar & 3.4625 & 0.1680 \\ |
440 |
< |
CRar & 3.555 & 0.1604 \\ |
441 |
< |
CA & 3.173 & 0.0640 \\ |
442 |
< |
HA & 2.746 & 0.0414 \\ |
432 |
> |
United Atom (UA) |
433 |
> |
&CH3 & 3.75 & 0.1947 & - & 3.54 & 0.2146 \\ |
434 |
> |
&CH2 & 3.95 & 0.0914 & - & 3.54 & 0.1749 \\ |
435 |
> |
&CHar & 3.695 & 0.1003 & - & 3.4625 & 0.1680 \\ |
436 |
> |
&CRar & 3.88 & 0.04173 & - & 3.555 & 0.1604 \\ |
437 |
> |
\hline |
438 |
> |
All Atom (AA) |
439 |
> |
&CT3 & 3.50 & 0.066 & -0.18 & 3.365 & 0.1373 \\ |
440 |
> |
&CT2 & 3.50 & 0.066 & -0.12 & 3.365 & 0.1373 \\ |
441 |
> |
&CTT & 3.50 & 0.066 & -0.065 & 3.365 & 0.1373 \\ |
442 |
> |
&HC & 2.50 & 0.030 & 0.06 & 2.865 & 0.09256 \\ |
443 |
> |
&CA & 3.55 & 0.070 & -0.115 & 3.173 & 0.0640 \\ |
444 |
> |
&HA & 2.42 & 0.030 & 0.115 & 2.746 & 0.0414 \\ |
445 |
> |
\hline |
446 |
> |
Both UA and AA & S & 4.45 & 0.25 & - & 2.40 & 8.465 \\ |
447 |
|
\hline\hline |
448 |
|
\end{tabular} |
449 |
|
\label{MnM} |
513 |
|
interfaces with UA model and different hexane molecule numbers |
514 |
|
at different temperatures using a range of energy fluxes.} |
515 |
|
|
516 |
< |
\begin{tabular}{cccccccc} |
516 |
> |
\begin{tabular}{ccccccc} |
517 |
|
\hline\hline |
518 |
< |
$\langle T\rangle$ & & $L_x$ & $L_y$ & $L_z$ & $J_z$ & |
519 |
< |
$G$ & $G^\prime$ \\ |
520 |
< |
(K) & $N_{hexane}$ & \multicolumn{3}{c}{(\AA)} & (GW/m$^2$) & |
518 |
> |
$\langle T\rangle$ & $N_{hexane}$ & Fixed & $\rho_{hexane}$ & |
519 |
> |
$J_z$ & $G$ & $G^\prime$ \\ |
520 |
> |
(K) & & $L_x$ \& $L_y$? & (g/cm$^3$) & (GW/m$^2$) & |
521 |
|
\multicolumn{2}{c}{(MW/m$^2$/K)} \\ |
522 |
|
\hline |
523 |
< |
200 & 266 & 29.86 & 25.80 & 113.1 & -0.96 & |
524 |
< |
102() & 80.0() \\ |
525 |
< |
& 200 & 29.84 & 25.81 & 93.9 & 1.92 & |
526 |
< |
129() & 87.3() \\ |
527 |
< |
& & 29.84 & 25.81 & 95.3 & 1.93 & |
528 |
< |
131() & 77.5() \\ |
529 |
< |
& 166 & 29.84 & 25.81 & 85.7 & 0.97 & |
530 |
< |
115() & 69.3() \\ |
531 |
< |
& & & & & 1.94 & |
532 |
< |
125() & 87.1() \\ |
533 |
< |
250 & 200 & 29.84 & 25.87 & 106.8 & 0.96 & |
534 |
< |
81.8() & 67.0() \\ |
535 |
< |
& 166 & 29.87 & 25.84 & 94.8 & 0.98 & |
536 |
< |
79.0() & 62.9() \\ |
537 |
< |
& & 29.84 & 25.85 & 95.0 & 1.44 & |
538 |
< |
76.2() & 64.8() \\ |
523 |
> |
200 & 266 & No & 0.672 & -0.96 & 102() & 80.0() \\ |
524 |
> |
& 200 & Yes & 0.694 & 1.92 & 129() & 87.3() \\ |
525 |
> |
& & Yes & 0.672 & 1.93 & 131() & 77.5() \\ |
526 |
> |
& & No & 0.688 & 0.96 & 125() & 90.2() \\ |
527 |
> |
& & & & 1.91 & 139() & 101() \\ |
528 |
> |
& & & & 2.83 & 141() & 89.9() \\ |
529 |
> |
& 166 & Yes & 0.679 & 0.97 & 115() & 69.3() \\ |
530 |
> |
& & & & 1.94 & 125() & 87.1() \\ |
531 |
> |
& & No & 0.681 & 0.97 & 141() & 77.7() \\ |
532 |
> |
& & & & 1.92 & 138() & 98.9() \\ |
533 |
> |
\hline |
534 |
> |
250 & 200 & No & 0.560 & 0.96 & 74.8() & 61.8() \\ |
535 |
> |
& & & & -0.95 & 49.4() & 45.7() \\ |
536 |
> |
& 166 & Yes & 0.570 & 0.98 & 79.0() & 62.9() \\ |
537 |
> |
& & No & 0.569 & 0.97 & 80.3() & 67.1() \\ |
538 |
> |
& & & & 1.44 & 76.2() & 64.8() \\ |
539 |
> |
& & & & -0.95 & 56.4() & 54.4() \\ |
540 |
> |
& & & & -1.85 & 47.8() & 53.5() \\ |
541 |
|
\hline\hline |
542 |
|
\end{tabular} |
543 |
|
\label{AuThiolHexaneUA} |
568 |
|
important role in the thermal transport process across the interface |
569 |
|
in that higher degree of contact could yield increased conductance. |
570 |
|
|
571 |
< |
[ADD Lxyz AND ERROR ESTIMATE TO TABLE] |
571 |
> |
[ADD ERROR ESTIMATE TO TABLE] |
572 |
|
\begin{table*} |
573 |
|
\begin{minipage}{\linewidth} |
574 |
|
\begin{center} |
577 |
|
interface at different temperatures using a range of energy |
578 |
|
fluxes.} |
579 |
|
|
580 |
< |
\begin{tabular}{cccc} |
580 |
> |
\begin{tabular}{ccccc} |
581 |
|
\hline\hline |
582 |
< |
$\langle T\rangle$ & $J_z$ & $G$ & $G^\prime$ \\ |
583 |
< |
(K) & (GW/m$^2$) & \multicolumn{2}{c}{(MW/m$^2$/K)} \\ |
582 |
> |
$\langle T\rangle$ & $\rho_{toluene}$ & $J_z$ & $G$ & $G^\prime$ \\ |
583 |
> |
(K) & (g/cm$^3$) & (GW/m$^2$) & \multicolumn{2}{c}{(MW/m$^2$/K)} \\ |
584 |
|
\hline |
585 |
< |
200 & -1.86 & 180() & 135() \\ |
586 |
< |
& 2.15 & 204() & 113() \\ |
587 |
< |
& -3.93 & 175() & 114() \\ |
588 |
< |
300 & -1.91 & 143() & 125() \\ |
589 |
< |
& -4.19 & 134() & 113() \\ |
585 |
> |
200 & 0.933 & -1.86 & 180() & 135() \\ |
586 |
> |
& & 2.15 & 204() & 113() \\ |
587 |
> |
& & -3.93 & 175() & 114() \\ |
588 |
> |
\hline |
589 |
> |
300 & 0.855 & -1.91 & 143() & 125() \\ |
590 |
> |
& & -4.19 & 134() & 113() \\ |
591 |
|
\hline\hline |
592 |
|
\end{tabular} |
593 |
|
\label{AuThiolToluene} |
620 |
|
However, when the surface is not completely covered by butanethiols, |
621 |
|
the simulated system is more resistent to the reconstruction |
622 |
|
above. Our Au-butanethiol/toluene system did not see this phenomena |
623 |
< |
even at $<T>\sim$300K. The Au(111) surfaces have a 90\% coverage of |
624 |
< |
butanethiols and have empty three-fold sites. These empty sites could |
625 |
< |
help prevent surface reconstruction in that they provide other means |
626 |
< |
of capping agent relaxation. It is observed that butanethiols can |
627 |
< |
migrate to their neighbor empty sites during a simulation. Therefore, |
628 |
< |
we were able to obtain $G$'s for these interfaces even at a relatively |
629 |
< |
high temperature without being affected by surface reconstructions. |
623 |
> |
even at $\langle T\rangle\sim$300K. The Au(111) surfaces have a 90\% |
624 |
> |
coverage of butanethiols and have empty three-fold sites. These empty |
625 |
> |
sites could help prevent surface reconstruction in that they provide |
626 |
> |
other means of capping agent relaxation. It is observed that |
627 |
> |
butanethiols can migrate to their neighbor empty sites during a |
628 |
> |
simulation. Therefore, we were able to obtain $G$'s for these |
629 |
> |
interfaces even at a relatively high temperature without being |
630 |
> |
affected by surface reconstructions. |
631 |
|
|
632 |
|
\subsection{Influence of Capping Agent Coverage on $G$} |
633 |
|
To investigate the influence of butanethiol coverage on interfacial |
703 |
|
its effect to the process of interfacial thermal transport. Thus, one |
704 |
|
can see a plateau of $G$ vs. butanethiol coverage in our results. |
705 |
|
|
706 |
< |
[NEED ERROR ESTIMATE, MAY ALSO PUT J HERE] |
707 |
< |
\begin{table*} |
708 |
< |
\begin{minipage}{\linewidth} |
709 |
< |
\begin{center} |
710 |
< |
\caption{Computed interfacial thermal conductivity ($G$) values |
711 |
< |
for the Au-butanethiol/solvent interface with various UA |
712 |
< |
models and different capping agent coverages at $\langle |
713 |
< |
T\rangle\sim$200K using certain energy flux respectively.} |
671 |
< |
|
672 |
< |
\begin{tabular}{cccc} |
673 |
< |
\hline\hline |
674 |
< |
Thiol & \multicolumn{3}{c}{$G$(MW/m$^2$/K)} \\ |
675 |
< |
coverage (\%) & hexane & hexane(D) & toluene \\ |
676 |
< |
\hline |
677 |
< |
0.0 & 46.5() & 43.9() & 70.1() \\ |
678 |
< |
25.0 & 151() & 153() & 249() \\ |
679 |
< |
50.0 & 172() & 182() & 214() \\ |
680 |
< |
75.0 & 242() & 229() & 244() \\ |
681 |
< |
88.9 & 178() & - & - \\ |
682 |
< |
100.0 & 137() & 153() & 187() \\ |
683 |
< |
\hline\hline |
684 |
< |
\end{tabular} |
685 |
< |
\label{tlnUhxnUhxnD} |
686 |
< |
\end{center} |
687 |
< |
\end{minipage} |
688 |
< |
\end{table*} |
706 |
> |
\begin{figure} |
707 |
> |
\includegraphics[width=\linewidth]{coverage} |
708 |
> |
\caption{Comparison of interfacial thermal conductivity ($G$) values |
709 |
> |
for the Au-butanethiol/solvent interface with various UA models and |
710 |
> |
different capping agent coverages at $\langle T\rangle\sim$200K |
711 |
> |
using certain energy flux respectively.} |
712 |
> |
\label{coverage} |
713 |
> |
\end{figure} |
714 |
|
|
715 |
|
\subsection{Influence of Chosen Molecule Model on $G$} |
716 |
|
[MAY COMBINE W MECHANISM STUDY] |
723 |
|
the previous section. Table \ref{modelTest} summarizes the results of |
724 |
|
these studies. |
725 |
|
|
701 |
– |
[MORE DATA; ERROR ESTIMATE] |
726 |
|
\begin{table*} |
727 |
|
\begin{minipage}{\linewidth} |
728 |
|
\begin{center} |
730 |
|
\caption{Computed interfacial thermal conductivity ($G$ and |
731 |
|
$G^\prime$) values for interfaces using various models for |
732 |
|
solvent and capping agent (or without capping agent) at |
733 |
< |
$\langle T\rangle\sim$200K.} |
733 |
> |
$\langle T\rangle\sim$200K. (D stands for deuterated solvent |
734 |
> |
or capping agent molecules; ``Avg.'' denotes results that are |
735 |
> |
averages of simulations under different $J_z$'s. Error |
736 |
> |
estimates indicated in parenthesis.)} |
737 |
|
|
738 |
< |
\begin{tabular}{ccccc} |
738 |
> |
\begin{tabular}{llccc} |
739 |
|
\hline\hline |
740 |
|
Butanethiol model & Solvent & $J_z$ & $G$ & $G^\prime$ \\ |
741 |
|
(or bare surface) & model & (GW/m$^2$) & |
742 |
|
\multicolumn{2}{c}{(MW/m$^2$/K)} \\ |
743 |
|
\hline |
744 |
< |
UA & AA hexane & 1.94 & 135() & 129() \\ |
745 |
< |
& & 2.86 & 126() & 115() \\ |
746 |
< |
& AA toluene & 1.89 & 200() & 149() \\ |
747 |
< |
AA & UA hexane & 1.94 & 116() & 129() \\ |
748 |
< |
& AA hexane & 3.76 & 451() & 378() \\ |
749 |
< |
& & 4.71 & 432() & 334() \\ |
750 |
< |
& AA toluene & 3.79 & 487() & 290() \\ |
751 |
< |
AA(D) & UA hexane & 1.94 & 158() & 172() \\ |
752 |
< |
bare & AA hexane & 0.96 & 31.0() & 29.4() \\ |
744 |
> |
UA & UA hexane & Avg. & 131(9) & 87(10) \\ |
745 |
> |
& UA hexane(D) & 1.95 & 153(5) & 136(13) \\ |
746 |
> |
& AA hexane & Avg. & 131(6) & 122(10) \\ |
747 |
> |
& UA toluene & 1.96 & 187(16) & 151(11) \\ |
748 |
> |
& AA toluene & 1.89 & 200(36) & 149(53) \\ |
749 |
> |
\hline |
750 |
> |
AA & UA hexane & 1.94 & 116(9) & 129(8) \\ |
751 |
> |
& AA hexane & Avg. & 442(14) & 356(31) \\ |
752 |
> |
& AA hexane(D) & 1.93 & 222(12) & 234(54) \\ |
753 |
> |
& UA toluene & 1.98 & 125(25) & 97(60) \\ |
754 |
> |
& AA toluene & 3.79 & 487(56) & 290(42) \\ |
755 |
> |
\hline |
756 |
> |
AA(D) & UA hexane & 1.94 & 158(25) & 172(4) \\ |
757 |
> |
& AA hexane & 1.92 & 243(29) & 191(11) \\ |
758 |
> |
& AA toluene & 1.93 & 364(36) & 322(67) \\ |
759 |
> |
\hline |
760 |
> |
bare & UA hexane & Avg. & 46.5(3.2) & 49.4(4.5) \\ |
761 |
> |
& UA hexane(D) & 0.98 & 43.9(4.6) & 43.0(2.0) \\ |
762 |
> |
& AA hexane & 0.96 & 31.0(1.4) & 29.4(1.3) \\ |
763 |
> |
& UA toluene & 1.99 & 70.1(1.3) & 65.8(0.5) \\ |
764 |
|
\hline\hline |
765 |
|
\end{tabular} |
766 |
|
\label{modelTest} |
797 |
|
|
798 |
|
However, for Au-butanethiol/toluene interfaces, having the AA |
799 |
|
butanethiol deuterated did not yield a significant change in the |
800 |
< |
measurement results. |
801 |
< |
. , so extra degrees of freedom |
802 |
< |
such as the C-H vibrations could enhance heat exchange between these |
803 |
< |
two phases and result in a much higher conductivity. |
800 |
> |
measurement results. Compared to the C-H vibrational overlap between |
801 |
> |
hexane and butanethiol, both of which have alkyl chains, that overlap |
802 |
> |
between toluene and butanethiol is not so significant and thus does |
803 |
> |
not have as much contribution to the ``Intramolecular Vibration |
804 |
> |
Redistribution''[CITE HASE]. Conversely, extra degrees of freedom such |
805 |
> |
as the C-H vibrations could yield higher heat exchange rate between |
806 |
> |
these two phases and result in a much higher conductivity. |
807 |
|
|
767 |
– |
|
808 |
|
Although the QSC model for Au is known to predict an overly low value |
809 |
< |
for bulk metal gold conductivity[CITE NIVSRNEMD], our computational |
809 |
> |
for bulk metal gold conductivity\cite{kuang:164101}, our computational |
810 |
|
results for $G$ and $G^\prime$ do not seem to be affected by this |
811 |
< |
drawback of the model for metal. Instead, the modeling of interfacial |
812 |
< |
thermal transport behavior relies mainly on an accurate description of |
813 |
< |
the interactions between components occupying the interfaces. |
811 |
> |
drawback of the model for metal. Instead, our results suggest that the |
812 |
> |
modeling of interfacial thermal transport behavior relies mainly on |
813 |
> |
the accuracy of the interaction descriptions between components |
814 |
> |
occupying the interfaces. |
815 |
|
|
816 |
|
\subsection{Mechanism of Interfacial Thermal Conductance Enhancement |
817 |
|
by Capping Agent} |
827 |
|
the velocity auto-correlation functions, which is used to construct a |
828 |
|
power spectrum via a Fourier transform. |
829 |
|
|
830 |
+ |
[MAY RELATE TO HASE'S] |
831 |
|
The gold surfaces covered by |
832 |
|
butanethiol molecules, compared to bare gold surfaces, exhibit an |
833 |
|
additional peak observed at a frequency of $\sim$170cm$^{-1}$, which |
840 |
|
combination of these two effects produces the drastic interfacial |
841 |
|
thermal conductance enhancement in the all-atom model. |
842 |
|
|
843 |
< |
[MAY NEED TO CONVERT TO JPEG] |
843 |
> |
[REDO. MAY NEED TO CONVERT TO JPEG] |
844 |
|
\begin{figure} |
845 |
|
\includegraphics[width=\linewidth]{vibration} |
846 |
|
\caption{Vibrational spectra obtained for gold in different |